Residual stress may be defined as stress that resides or remains within a workpiece following the application of one or more manufacturing operations. Manufacturing operations that induce residual stress in a workpiece may include any process where heat, pressure and/or energy are applied to the workpiece. For example, a machining operation may be performed on a metallic (e.g., aluminum, titanium) workpiece in order to shape the workpiece into its intended configuration. During the machining operation, a rotating cutting tool may be placed in direct contact with the workpiece. The cutting tool is moved along the workpiece surface at a desired feed rate to remove layers of the metallic material. The removal of metallic material by the cutting tool causes local yielding and plastic strain in the outer surface layer of the workpiece inducing residual stress in the machined surface.
The magnitude of the machining-induced residual stress is typically highest at the outer surface of the workpiece and diminishes to zero within a relatively shallow depth. In addition, depending upon the manufacturing operation, the residual stress may be compressive, tensile or a combination of compressive and tensile through the thickness of the residual stress layer. Residual stress that is predominantly compressive may enhance the strength and fatigue properties of the workpiece. Post-machining operations such as shot-peening may be performed on a machined surface to impart compressive residual stress into the outer layer to enhance the fatigue strength and thereby extend the fatigue life of a workpiece. Residual stress that is predominantly tensile is generally detrimental to fatigue strength and may shorten the fatigue life of the workpiece.
Distortion is another undesirable effect of residual stress that may occur in a workpiece following a manufacturing operation. Distortion may be characterized as in-plane distortion or out-of-plane distortion. In-plane distortion includes expansion or contraction of the workpiece along a direction parallel to the plane of the workpiece surface. Out-of-plane distortion includes displacement in the form of twisting and/or bending of the workpiece surface along a direction perpendicular to the surface.
Although the depth of machining-induced residual stress in a workpiece is typically shallow (e.g., 0.004 to 0.020 inch), out-of-plane distortion has a more noticeable effect on relatively thin metallic cross-sections that are less resistant to bending as compared to thicker cross-sections that are more resistant to bending. Unfortunately, moderate distortion may result in expensive and time-consuming inspection and reworking to bring the workpiece within design tolerances. Excessive distortion may lead to scrapping of the workpiece and fabrication of a replacement.
The parameters of the machining operation have an effect on the magnitude, type (i.e., compressive or tensile in the surface) and through-thickness distribution of residual stress in a workpiece. For example, during high-speed machining, a cutting tool may be rotated at relatively high speed and/or the cutting tool may be driven into the workpiece at a relatively high feed rate which, depending on the machining parameters, may result in non-optimal machining of the workpiece. Non-optimal machining is believed to increase residual stress and distortion in a workpiece as compared to more conventional machining. Other machining parameters that may affect the magnitude and orientation of machining-induced residual stress include, without limitation, the geometric parameters of the cutting tool, whether the end of the cutting tool is used as the cutting surface to remove material or whether the sides of the cutting tool are used as the cutting surface, and the sharpness of the cutting tool. The composition or alloy of the workpiece material may also affect the character of the residual stress.
The known methods for predicting residual stress in a workpiece generally include performing a finite element analysis by applying an estimate of a predicted residual stress to a finite element model of the workpiece. The predicted residual stress is an estimate of the residual stress that may occur in the workpiece as a result of the manufacturing operation. Unfortunately, current systems and methods of estimating residual stress are generally inadequate. Furthermore, current methods of verifying the accuracy of the estimated residual stress due to a manufacturing operation are time-consuming. For example, current methods of verifying residual stress estimates require measuring residual stress in the machined workpiece and comparing the measurements to the predictions from the model solved using the finite element method. The parameters of the manufacturing operation are then adjusted or the workpiece may be redesigned in an attempt to reduce or eliminate the residual stress and distortion in the workpiece. The process is repeated in an iterative manner until the residual stress in the physical workpiece falls within acceptable limits. Unfortunately, the process of iteratively adjusting the machining parameters, fabricating a new workpiece, measuring the residual stress in the new workpiece, and then re-adjusting the machining parameters is time consuming and costly.
As can be seen, there exists a need in the art for a method of validating predictions of residual stress that may occur in a workpiece resulting from a selected manufacturing operation without fabricating the workpiece and measuring the residual stress.